1. Field of the Invention
My invention relates to digital halftoning, specifically to the replacement of transparent pixels within a monochrome digital halftone cell with colorized pixels.
2. Description of Prior Art
Digital halftones heretofore have reproduced color by utilizing four monochrome files: one each for three primary colors and one for black. Each of the four files is of comparable size and complexity, making the size and complexity of the full image difficult to transmit and store efficiently.
Nearly 150 years ago James Clerck Maxwell discovered that any color may be created by mixing three primary colors. The science of colorimetry steadily has evolved to the present. The publication of color space standards by the International Commission on Illumination (CIE) beginning in 1931 has initiated much of today's understanding of color.
Color representation may be additive (light emitting), as exemplified by a computer monitor, or subtractive (light reflecting), as exemplified by a printed page. Additive color is considered best represented by the mixing of the primary colors red, blue and green (the RGB model). Subtractive color is considered best represented by the mixing of the primary colors cyan, magenta and yellow (the CMY model).
While color may be represented by three determinants, the addition of a fourth determinant has improved results. For the sake of simplicity, this determinant may be called intensity of light or luminance. Luminance in the additive color model is contained in an Alpha channel and is known as RGBα. Luminance in the subtractive color model is contained in a Black channel (K) and is known as CMYK.
In the digital realm, the RGBα, CMYK or other four-channel model will result in increased file size if each is represented by the same number of bits.
In U.S. Pat. No. 5,504,846, “Method and Apparatus for Improved Area Demarcation in Bit Mapped Image Derived from Multi-Color Bit Mapped Image,” Fisher attempted to improve a subtractive printed image derived from an additive screen image by combining adjacent areas of the same color. However, his method necessitated additional color/dither (halftone) patterns, increasing file size, differentiating it from my proposed method.
In U.S. Pat. No. 5,991,512, “Ink Relocation for Color Halftones,” Shaked, et. al., attempted to reduce color halftone noise by selectively moving color areas. However, their method resulted in a blurring side-effect that necessitated enhancement procedures including suppression of their method in areas of fine detail as well as halftone edge sharpening. Such enhancement procedures also increase file size, differentiating their method from my proposed method.